1. Technical Field
The present invention relates to a control system for a four-wheel drive vehicle that can freely stop a propeller shaft on which an automatic brake device is installed which causes automatic braking to be performed in response to an obstacle or a sharply curved road in front of the vehicle.
2. Related Art
Heretofore, in a four-wheel drive vehicle in which a driving force is transmitted through a propeller shaft, which can be freely stopped, from a main drive shaft to an auxiliary shaft, in order to reduce a travelling resistance, it has been necessary to firstly synchronize the propeller shaft with the wheels by applying a torque to the propeller shaft when a two-wheel drive operation, in which the propeller shaft is stopped, must be switched to a four-wheel drive operation while the vehicle is traveling. Since the rotational synchronization torque applied to the propeller shaft acts as a brake torque on a drive system, when the synchronization torque becomes large, the vehicle is subject to unpleasant and unwanted deceleration and shock (back-and-forth jerking). If an attempt is made to suppress generation of the brake torque, the two-wheel drive operation cannot be rapidly switched to the four-wheel drive operation. As a result, there is a great problem in a trade-off between reduction of the braking torque and reduction of a switching period. A technology in a four-wheel drive vehicle for switching a two-wheel drive operation to a four-wheel drive operation is disclosed in, for instance, Japanese Unexamined Patent Application Publication (JP-A) No. 2010-100280. The vehicle having such a technology includes a first clutch that transmits a variable part of a driving torque to an auxiliary accelerator of the vehicle and a second clutch that deactivates a propeller shaft provided between the first and second clutches when the first clutch is uncoupled. The second clutch is coupled in accordance with a wheel slip detected in a main accelerator and the propeller shaft in the deactivated state is accelerated before coupling of the second clutch.
However, in the four-wheel drive vehicle disclosed in JP-A No. 2010-100280, even if smooth rotational synchronization is realized for the propeller shaft, an operation in which the propeller shaft is accelerated by consuming a vehicle travelling (moving) energy or an additional fuel results in an energy loss that runs counter to an aim of increasing fuel efficiency by adopting a propeller shaft that can freely stop.
In current vehicles, various automatic braking systems have been developed and put into use in order to enhance safety and reduce the burden on the driver. Such automatic braking systems are actively adopted in four-wheel drive vehicles. In a four-wheel drive vehicle that can freely stop a propeller shaft as described above, if it is possible for the automatic braking system to absorb an energy necessary to synchronize the main drive shaft and the propeller shaft from a kinetic energy of the vehicle to be consumed and then convert and utilize the energy, it is preferable to effectively utilize the kinetic energy of the vehicle to be consumed in some form. In circumstances where automatic braking is performed in order to prevent a collision between a vehicle and an obstacle or to prevent the vehicle from deviating from a traffic lane, there is high possibility of tire grip reaching its limit. Accordingly, it would be effective in view of safety to switch the two-wheel drive operation to the four-wheel drive operation in advance. Furthermore, if an operation of a main brake is reduced in consideration of the deceleration applied to the vehicle by rotational synchronization, it will be possible to reduce a heat load acting on the brake.